The invention relates to a bone screw for connecting a bone with a rod.
A bone screw of this kind is disclosed in the German Patent Specification 39 23 966. This known bone screw comprises a screw member having a threaded portion and a head, said head having a spherical segment-shaped portion, a cylindrical receiver member for receiving the head of the screw member and the rod, the receiver member having a first end, a first bore provided at the first end for passing the threaded portion therethrough, a hollow spherical portion receiving the head at a position inwards adjacent to the bore, a second bore which is open towards the side opposite to the first bore for inserting a screw member and the head thereof. The rod to be connected to the bone screw is threaded and connected by means of two nuts provided on the threaded rod and engaging the exterior surface of the receiver member.
The European Patent Specification 0 465 158 discloses an anchoring member having a shaft to be anchored in the bone and a head for connection with a rod. The head has a substantially U-shaped cross-section with a base which is connected to the shaft and with two free legs forming a channel for receiving the rod. A first internal thread extends in direction of the legs at the inner side thereof, and a locking member as well as a further member embracing the outside of the legs are provided. The locking member and the embracing member are connected through an intermediate insert which is screwed into the internal thread of the two free legs. This known anchoring means requires that first the intermediate piece is locked with the embracing member and only thereafter the locking member is screwed in. The European Patent Specification 0 443 892 discloses an apparatus wherein an outer ring is placed around free legs of a head and the outer ring comprises a tongue engaging the channel. A locking member acting on the tongue when locked is screwed into an internal thread. A separate movement of the embracing member and of the locking member for manipulating the apparatus is not provided. A lock of the screws is also not provided and the fixation of a bent rod is not possible.